1. Field of the Invention
The invention concerns the beam welding of engineering components made of hardenable steel. The invention is useful for all components joinable by beam welding methods, which components are subjected to mechanically, cyclically, or dynamically high loads. Because of a local wear load, these components consist at least partially of hardenable steels or which are hardened and annealed because of their high mechanical load. The invention can be used particularly advantageously for the production of extremely varied, in particular rotationally symmetric power transmission elements, pressure-exposed hollow parts, hydraulic rams, valves, etc. A preferred area of application of the invention is motor vehicle and mechanical engineering, primarily automotive engineering.
2. Description of Background Information
Carbon steels with a carbon content of Cxe2x89xa70.25% and low-alloy steels with carbon contents of Cxe2x89xa70.20% are only weldable to a limited extent by commercial means, as the increased hardening in the welding and heat affected zone caused by the carbon and by various alloy elements results in cracks. The hardening and subsequent crack formation occurs through the formation of martensite or lower bainite, which are only slightly deformable, and slightly (or not at all) self-annealed, which are incapable of plastically mitigating the high transient stresses occurring during cooling.
A method to prevent inadmissible hardening, and thus also crack formation with conventional welding methods requires the bulk preheating of the components. For carbon steels with carbon contents C of 0.3%xe2x89xa6Cxe2x89xa60.45%, preheating temperatures of 150 to 275xc2x0 C. are recommended (see, e.g., J. Ruge xe2x80x9cManual of Welding Technologyxe2x80x9d, Vol. 1, xe2x80x9cMaterialsxe2x80x9d, 3rd ed., Springer-Verlag, Heidelberg 1991, ISBN-3-540-52697-8, p. 126, p. 144]. For low-alloy steels, the necessary preheating temperature may rise to temperatures as high as 400xc2x0 C. (see, e.g., Quality and Stainless-Steels of the GDR, Leipzig, 1972, Vol. 1).
However, for many components, in particular mass-produced components, conventional welding methods have disadvantages with regard to welding speed, component distortion, unit costs, and after-treatment expense. These shortcomings are from relatively low power densities, which result in relatively low heating speeds, relatively high introduction of heat, and large welding seam volumes.
Beam welding methods such as laser or electron beam welding avoid these disadvantages by using power densities which are as many as a few orders of magnitude higher. However, such methods result in a higher hardening of the fusion heat affected zone, with a corresponding higher susceptibility to cracking of the welding seams. This shortcoming severely restricts the palette of beam weldable steels, since the limit of carbon and alloy element content which can be managed without cracking drops.
The effects of these drawbacks are increased, in that conventional methods of bulk preheating can be integrated into automated beam welding systems only with difficulty. Short cycle times are too expensive, and result in a deterioration of welding seam quality from oxidation of the joint.
This drawback is caused by extremely high quenching speed, which is clearly less than the t8/5-times.
According to the patent J-1-40194 entitled xe2x80x9cLaser Beam Welding method for joining materialxe2x80x9d, a method is known for laser welding non-hardenable metal sheets to reduce the quenching speed with laser welding by process-integrated post-heating. For this, a high frequency inductorxe2x80x94located behind the laser welding head in terms of the feed rate and fixedly connected theretoxe2x80x94is guided at the laser welding rate at a distance from the surface determined by the focus distance of the laser beam and the geometric layout. Because of the use of the high frequency, a narrow strip is heated on both sides of the I-welding seam of the welded sheet, and thus the quenching speed is reduced. The object of the method is increased ductility and improvement of the workability of the metal sheet.
For this process, the metal sheet is heated to approximately 1000xc2x0 C. However, because of the use of high frequency, the process is restricted in application to thin metal sheets. By changing the feed rate of the inductor (which can be changed only to the same extent as the laser welding rate defined by other criteria), the temperature, the inductor length and width, the cooling speed may be varied within a relatively narrow framework.
A drawback of this method is that it can be used only for thin metal sheets and only very limited for hardenable steel. It is therefore not usable for power transmission elements or mechanically functional components.
The nature of the above drawback is that the superimposed temperature time cycle of the inductive post-heating cannot be adjusted, or at least cannot be adjusted for all depth zones, within the welding seam to the requirements necessary to avoid hardening with beam welding of hardenable steels. Specifically, since the heating depth is not adjustable to the necessary welding seam depth, the cooling speed cannot be selected adequately small (or at least not for the entire welding seam depth). The high peak temperature of the post-heating cycle therefore destroys the defined normalized or tempered structure. Because the heating depth with the inductive application of energy is too little with a high frequency, heat energy is not introduced until after termination of the welding process. The and the heat penetration rate into the component is comparatively small relative to the laser welding speed. The limiting isotherm of adequately higher annealing temperatures reaches the deeper regions of the welding seam only after periods of time in which the temperature has already fallen below the MS-temperature. Consequently, hardening occurs. The high peak temperature of approximately 1000xc2x0 C., which is higher than the austenitizing temperature and which results from the relative heating depth which is slight with reference to the component thickness as well as the relatively high quenching speed resulting from the high feed rate leads moreover to the danger of new hardening even in regions outside the HAZ of the welding zone.
The present invention provides a method by which hardenable steels may be effectively welded crack-free and without troublesome hardening.
The present invention uses a transient temperature field and a method to produce it which can be readily integrated into processes and which is adaptable even for hardenable steels with relatively high critical cooling times and relatively deep welding seams. The entire welding seam has has an adequately low cooling speed and the normalized or tempered basic structure of the initial state outside the welding zone and the heat affected zone is not damaged.
The short-time heat treatment is performed as the sole preheating. The heating depth before the beginning of the beam welding ti2 is selected such that it reaches 1.0 to 5.0 times the welding seam depth. The energy exposure time itself, the induction frequency, and, to a small extent, the peak temperature Tmax of the preheating cycle serve as free parameters for the setting of the heating depth ti2.
The peak temperature is selected in a temperature range from 620 Kxe2x89xa6Tmaxxe2x89xa6TZxe2x88x9230 K., whereby the temperature TZ depends on the starting structure of the materials to be joined. In the pearlitic state, TZ corresponds to the temperature at which a perceptible spherodizing of cementite begins within a period of 1 second to 100 seconds. In the tempered initial state, it corresponds to the temperature of the preceding annealing treatment. The selection of this temperature guarantees the best conditions for the subsequent cooling cycle without damaging the structure.
It is crucial for the welding result, and particularly advantageous for the process design, that the quenching time xcfx84k is adjusted by the use of the natural cooling capacity of the component. The relative depth and the width of the preheating zone serve as free parameters for adjustment of the concrete value xcfx84k.
The heating depth ti1 may be adjusted after the end of the energy exposure cycle by the frequency of the induction generator. Thus, the energy exposure period xcfx84S and the cooling time xcfx84A are reduced. Moreover, additional degrees of freedom are obtained for the adaptation of the duration of the energy exposure xcfx84S to the welding time xcfx84L, which is essential for cycle time optimization.
Through the selection of the energy exposure period xcfx84S, the maximum temperatures Tmax, the cooling times xcfx84a, and the geometric dimensions of the preheating zone bi1 and ti1, it is possible to complete the preheating cycle fully before the welding cycle. This permits more free space in the layout of corresponding welding machine designs.
The maxima of the temperature field for preheating may be placed at distances of one to three times the wall thickness of the component. Thus, in the case of critical materials with particularly high t8/5-times, a particularly low cooling speed is achieved at the joint with constant maximum temperatures of the preheating cycle.
Leakage of the air from inside closed hollow parts through the joining gap can be prevented, although the pressure of the air is increased by the preheating. Thus, very good welding seam quality is achieved without additional vent holes, which are often undesirable and require additional production expense, and hose pores reaching all the way to the surface of the welding seam are avoided.
Necessary hardening or tempering treatments possibly following the welding operation may be integrated into the heat treatment cycle of the welding by using simple process technology and optimum utilization of energy.
For production technology reasons, the individual parts to be joined are easier to harden than finished welded structures.